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DNA. We talk about it so much---it is the ultimate director for cells and it codes for
your traits. It’s a major component of what makes you, you. When you have a really important
molecule like DNA that is ultimately responsible for controlling the cell…it would make sense
that when you make another cell (like in mitosis), you would also have to get more DNA into that
cell. And that introduces our topic of DNA replication, which means, making more DNA.
First let’s talk about where and when.First where---it occurs in the nucleus. If the cell
has a nucleus. Remember, not all cells have a nucleus. This video clip is actually going
to focus on the types of cells that do have a nucleus though known as eukaryote cells.
Prokaryotes, which are cells that lack a nucleus, do things a little differently. Next When
does this happen---this typically happens during a stage known as interphase. Interphase
is when a cell’s growing, it’s carrying out cell processes, and it’s replicating
its DNA. You know what it’s not doing at the exact same time? Dividing. You don’t
want a cell to be replicating DNA and dividing at the exact same time. That’s a little
bit too much multitasking. So DNA replication does not happen during cell division (aka
mitosis). In fact, the cell replicates its DNA before division processes like mitosis
and meiosis. Because once you make that new cell, you better have DNA to put in there.
I think DNA replication would actually make a great video game. It’s actually quite
exciting. I’m going to introduce the key players in DNA replication so that you can
get some background information. The majority of these key players that I’m going to introduce
are enzymes. In biology, when you see something end in –ase, you might want to check as
it is very possible that it’s an enzyme. Enzymes have the ability to speed up reactions
and build up or break down the items that they act on. So here we go with the key players.
Helicase- the unzipping enzyme. If you recall that DNA has 2 strands, you can think of helicase
unzipping the two strands of DNA. Helicase doesn’t have a hard time doing that. The
hydrogen bonds that hold the DNA strands together is pretty weak compared to other kinds of
bonds. DNA Polymerase- the builder. This enzyme replicates DNA molecules to actually build
a new strand of DNA. Primase- The initializer. With as great as DNA polymerase is, poor DNA
polymerase can’t figure out where to get started without something called a primer.
Primase makes the primer so that DNA polymerase can figure out where to go to start to work.
You know what’s kind of interesting about the primer it makes? It’s actually a piece
of RNA. Ligase- the gluer. It helps glue DNA fragments together. More about why you would
need that later. Don’t feel overwhelmed. We’ll go over the sequence in order. Please
keep in mind, that like all of our videos, we tend to give the big picture but there
are always more details to every biological process. There is more involved than what
we cover. DNA replication starts at a certain part called the origin. Usually this part
is identified by certain DNA sequences. There can be multiple origins within the DNA strand.
At the origin, helicase (the unzipping enzyme) comes in and unwinds the DNA.
SSB proteins (which stands for single stranded binding proteins) bind to the DNA strands to keep
them separated. Primase comes in and makes RNA primers on both strands. This is really
important because otherwise DNA polymerase won’t know where to start.
Now comes DNA Polymerase. Remember, it’s the important enzyme that adds DNA bases.
Now you have 2 strands right? But they’re not identical.Remember they complement each other. They
also are anti-parallel so they don’t really go in the same direction.
With DNA, we don't say it goes North or South. The directions for the DNA strands are a little different.
We say that DNA either goes 5’ to 3’ or 3’ to 5’. What in the world does that
mean? Well the sugar of DNA is part of the backbone of DNA. It has carbons. The carbons
on the sugar are numbered right after the oxygen in a clockwise direction. 1’, 2’
3’, 4’ and 5.’ The 5’ carbon is actually outside of this ring structure. Now you do
the same thing for the other side but keep in mind this strand is flipped just because
DNA strands are anti-parallel to each other. So let’s count these---again, clockwise
after the oxygen. 1’, 2’ 3’, 4’ 5’. And the 5’ is out of this ring. This strand
on the left runs 5’ to 3’ and the strand on the right here runs 3’ to 5’. Well,
it turns out that DNA polymerase can only works in the 5’ to 3’ direction. So…the
strand that runs 5’ to 3’ is fine. It is called the leading strand. But the other
strand will make it a little tricky. DNA polymerase can only go in the 5’ to 3’ direction.
(NOTE: Reads in 3' to 5' direction). Primase has to set a lot of extra primers down to
do that as shown here. It takes longer too. This strand is called the lagging strand which
is pretty fitting.On the lagging strand, you tend to get little fragments of synthesized
DNA. These are called Okazaki fragments. Okazaki. What an amazing name. The primers have to
get replaced with DNA bases since the primers were made of RNA. Ligase, the gluing enzyme
as I like to nickname it, has to take care of the gaps in the Okazaki fragments.Now at
the end, you have two identical double helix DNA molecules from your one original double
helix DNA molecule. We call it semi-conservative because the two copies each contain one old
original strand and one newly made one. One last thing. Surely you have had to proofread
your work before to catch errors? Well, you definitely don’t want DNA polymerase to
make errors. If it matches the wrong DNA bases, then you could have an incorrectly coded gene…which
could ultimately end up in an incorrect protein---or no protein. DNA polymerase is just awesome…it
has proofreading ability. Meaning, it so rarely makes a mistake. Which is very good. That’s
it for the amoeba sisters and we remind you to stay curious!
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